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A class of vertebrates composed of four living orders, the turtles or Chelonia, the tuatara or Sphenodonta, the lizards and snakes or Squamata, and the crocodylians or Crocodylia. Numerous extinct orders are also known. The group first appeared in the Carboniferous and underwent a culminating evolutionary radiation in the Mesozoic, often called the age of reptiles. Although the major portion of the class is now extinct, several Recent groups, particularly the Squamata, are very successful, and there are approximately 5000 living species of reptiles as compared to about 4000 living mammals.
The reptiles are the most primitive of the completely terrestrial vertebrates and are consequently the first to exhibit amniote features. Reptile eggs are covered by a complex series of protective layers, including a leathery or calcareous shell. A rich supply of food material in the form of yolk is deposited inside the ovum to furnish food for the developing embryo. A series of protective extraembryonic membranes, the serosa and amnion, appears later in embryogenesis to protect the embryo from water loss and shock. A third such membrane, the allantois, functions as a storage sac for nitrogenous wastes. The serosa and allantois usually fuse to form a respiratory structure. Gaseous exchanges take place across the shell and seroallantoic membrane between the outside air and the blood vessels of the allantois. These adaptations allowed the reptile egg to be deposited on land, undergo its development there, and hatch into a fully developed form without a gilled larval stage. Most reptilian eggs are buried in the soil or in rotting vegetation out of direct sunlight. See Amniota
Paleoherpetology, the study of fossil reptiles, is especially important for two reasons. First, the class Reptilia lies at the center of vertebrate history; the reptiles evolved from the amphibians (which themselves had originated from the fishes), and both birds and mammals evolved from the reptiles. Thus reptiles are concerned in three of the four major “jumps” (the class-to-class transitions) in vertebrate evolution. The distinction between the living representatives of two successive classes is always very clear, being based on a number of features of their anatomy, physiology, and embryology; the distinction between their fossil members, however, is inevitably less clear, not only because the distinction must be based almost entirely upon characters of the skeleton but also because there must have been animals with a mixture of the characters of both classes during the transitional period. These help elucidate the reasons for the jumps and the precise mechanism by which each occurred. See Amphibia, Animal evolution
Second, the Reptilia were the dominant class of land vertebrates (and were also important in the sea and in the air) during a very long period of the Earth's history. Knowledge of the extinct reptiles, their morphology and their habits, is vital to an understanding of the life of those times, of how the animals and plants and the physical environment reacted upon each other (paleoecology).
a class of vertebrates. Modern reptiles are relicts of a rich and varied reptilian population that flourished during the Mesozoic.
Reptiles occupy an evolutionary position after their amphibian ancestors. They represented the final step in the adaptation of vertebrates to life on dry land. Reptilian eggs are large, rich in yolk, and protected from desiccation by a thick membrane. The embryos are similar to those of birds and mammals in that they have membranes, including an amnion. Hence, reptiles are grouped with birds and mammals in the group Amniota (higher vertebrates).
Reptiles are much more highly organized than amphibians, but they represent the most primitive group of higher vertebrates. The reptilian brain is less developed than that of birds and mammals. The circulatory system in modern reptiles is mixed, and the body temperature is not constant. The skin is covered with horny scales or plates or with hairs (pterosaurs), which protect the body from desiccation and injury; only the skin of certain aquatic turtles lacks protection. In most reptiles, glands are not present in the skin. The skeleton is almost completely ossified.
The skull of modern reptiles articulates with the spine by a single occipital condyle—a characteristic shared with birds. Most reptiles have numerous membrane bones that cover the cartilaginous cranium above and laterally. In the temporal region the membrane bones are somewhat reduced and form openings that are divided by one or two bony bridges known as visceral arches. The lower jaw articulates with the cranium by means of the quadrate bone.
Reptiles are more mobile than amphibians, owing to the progressive development of their skeleton and musculature. The vertebral column (excluding snakes and limbless lizards) is divided into cervical, thoracic, sacral, and caudal sections. The thoracic vertebrae bear long ribs, which unite with the sternum to form the thorax. Modern reptiles have two sacral vertebrae. Intertarsal articulation is characteristic of the hind legs.
The hemispheres of the forebrain have a small layer of gray matter, which forms the true cerebral cortex. The respiratory system is more differentiated than that of amphibians. It includes a trachea, bronchi, and alveolar lungs. As in all Amniota, respiration is effected by movement of the ribs. Only in turtles, owing to the bony shell, are the lungs ventilated by swallowing air and rhythmically moving the flank muscles. In modern reptiles the heart is three-chambered; there are two auricles and one ventricle, which is partially or completely (in crocodiles) divided by a septum. Three vessels depart from the ventricle: the pulmonary artery and the right and left aortas. Reptiles have a metanephros, as do all other Amniota. The ureters open into the cloaca. Most reptiles have a urinary bladder; in some (crocodilians, snakes, and some lizards) it is underdeveloped. The males (except tuataras) have a copulatory organ; in snakes and lizards it is paired.
Modern reptiles include Chelonia (turtles), Crocodylia (crocodilians), Rhyncocephalia (tuatara), and Squamata (lizards, snakes, and Amphisbaenidae—including Amphisbaena and Trogonophis). The overwhelming majority of reptiles are terrestrial, preferring open landscapes warmed by the sun, including waterless deserts almost totally lacking in vegetation. All crocodilians and many turtles inhabit lakes, rivers, and swamps, and some turtles and snakes are permanent inhabitants of seas.
The activity of modern reptiles, owing to their inconstant body temperature, to a large degree depends on the temperature of the environment. When the body temperature cools to 8° to 6°C, most reptiles stop moving. Most can withstand strong sunlight and can raise their body temperature to 40°C. To avoid overheating, they move to the shade or hide in burrows. Seasonal climatic changes greatly influence the activity of reptiles. In temperate climates, reptiles spend the winter in hibernation; under conditions of dry heat they spend the summer inactive.
Most reptiles are carnivorous. Crocodiles and sea and freshwater turtles feed mainly on fish and aquatic invertebrates. Nearly all lizards feed predominantly on insects. Some snakes have adaptations enabling them to kill and swallow prey that is many times their own size. Boa constrictors and some poisonous snakes feed mainly on relatively large mammals and birds. Some lizards, agamas, and iguanas eat both plant and animal substances. Some reptiles are exclusively herbivorous (terrestrial tortoises).
The typical reptilian mode of reproduction is oviparous. However, some species are ovoviviparous or viviparous. The turtle and crocodilian eggs are covered with a hard calcareous shell, under which is an albumen membrane (as in a bird’s egg). Lizard and snake eggs have a soft, parchmentlike shell and no albumen membrane. The incubation period ranges from one or two months to a year or more (tuatara). The young break through the eggshell by means of an egg tooth or a special egg horny tooth (in turtles). Some lizards and snakes are ovoviviparous; the fertilized eggs remain in the oviducts, where the embryos finish their development. The young hatch immediately after the eggs are laid or even earlier.
True viviparity is characteristic of some lizards (skinks). The eggs that develop in the oviducts have no membranes, and the embryo, by means of the blood vessels of the yolk sac and allantois, unites with the vessels of the oviduct (rudimentary placenta) and obtains nourishment from the maternal body. Parthenogenesis, by which reproduction occurs without the participation of males, is characteristic of some lizard species. As a rule, modern reptiles do not tend their young.
The most ancient and primitive group of reptiles were Cotylosauria, which appeared in the Middle Carboniferous and were structurally similar to the labyrinthodonts, a group of ancient amphibians. The Cotylosauria included lizardlike insectivores and predators, as well as gigantic herbivorous forms. The Hercynian folding of the Late Paleozoic, which caused the drying up of extensive swampy lowlands, promoted a substantial increase in the range of reptiles. By the end of the Carboniferous, theromorphs appeared, which subsequently gave rise to mammals in the Triassic. In the Permian, theromorphs spread to all continents and became the prevailing reptilian group. Among them were predatory forms—theriodonts— and herbivorous forms— dicynodonts. Reptiles attained enormous diversity in the Mesozoic, which is aptly called the reptilian age. At that time they inhabited not only dry land but also fresh and marine waters. Flying forms also appeared. All groups of reptiles, including some that have been preserved to this day, developed during the Mesozoic, Cotylosauria became extinct during the Triassic. The diversity of theromorphs, whose last representatives are known from the Middle Jurassic, was sharply curtailed. It was during the Triassic that turtles, the first lizards, the tuatara, Ichthyopterygia, Sauropterygia, and Placodonta appeared. Placodonts resembled turtles but differed sharply from them in skull and shell structure. Ichthyopterygia and Sauropterygia inhabited seas and were especially numerous in the Jurassic, when the seas covered substantial areas. They became extinct in the Cretaceous. The youngest group of reptiles—snakes—appeared in the Cretaceous.
The most extensive and diverse group of Mesozoic reptiles were the Archosaura, most of which were adapted to bipedal locomotion (they moved on their hind legs). This form of locomotion characterized typical Pseudosuchia, which gave rise to birds and to the remaining groups of Archosaura—some Phytosauria, dinosaurs, true crocodiles, and pterosaurs. All these groups appeared at the end of the Triassic or in the Jurassic. Some dinosaurs preserved bipedal locomotion (predators and ornithopods), whereas others returned to quadruped locomotion (sauropods, stegosaurs, ankylosaurs, and Ceratopsia). The dinosaurs, which inhabited the water or lived along shores, included the largest terrestrial vertebrates, both herbivores (sauropods) and predators (Carnosauria). Crocodilians returned secondarily to quadruped locomotion; their ancient representatives are very similar to Pseudosuchia. The pterosaurs, which were adapted to flight and therefore acquired specialized features, deviated most markedly from the Pseudosuchia.
The causes for the extinction at the end of the Mesozoic of many groups of reptiles that had flourished earlier are not clear. It was apparently associated with abrupt changes in the earth’s surface and changes in the biotic environment. The Alpide folding caused considerable desiccation of the continents and thus deprived the majority of ancient reptiles of their natural habitat.
Reptiles are of little value to humans. The skins of crocodiles, large snakes, and lizards are used in the manufacture of suitcases, briefcases, and footwear. The flesh and eggs of some turtle species are used as food; in some tropical countries the flesh of crocodiles, such large lizards as monitors and iguanas, and some large snakes is used as food. The majority of lizards are valued for their services in insect control, and many snakes destroy harmful rodents. Only poisonous snakes can cause great harm, especially in tropical countries, where a substantial number of people and domestic animals die or become seriously ill from snake bites. At the same time, the toxins of many species of snakes are widely used to manufacture medicinal preparations. Large crocodiles cause losses to animal husbandry and even endanger human lives in some places. Many turtles harm commercial fishing. Some reptiles, especially some large ones (crocodiles and some turtles), are sharply decreasing in number and are on the verge of extinction.
REFERENCEBrehm, A. Zhizn’zhivotnykh, 4th ed., vols. 4–5. St. Petersburg, 1914. (Translated from German.)
Zhizn’ zhivotnykh, vol. 4, part 2. Moscow, 1969.
Nikol’skii, A. M. Presmykaiushchiesia, vols. 1–2. Petrograd, 1915–16. (Fauna Rossii i sopredel’nykh stran.)
Terent’ev, P. V., and S. A. Chernov. Opredelitel’ presmykaiushchikhsia i zemnotodnykh, 3rd ed. Moscow, 1949.
Terent’ev, P. V. Gerpetologiia. Moscow, 1961.
Osnovy paleontologii: Zemnovodnye, presmykaiushchiesia i ptitsy. Moscow, 1964.
Romer, A. S. Vertebrate paleontology, 3rd ed. Chicago-London, 1966.
Biology of the Reptilia, vols. 1–4. London-New York, 1969–74.
Bellairs, A. The Life of Reptiles, vols. 1–2. New York, 1970.
I. S. DAREVSKU AND N. V. SHIBANOV